Chemical-looping combustion (CLC) is a flameless combustion technology that requires no direct contact between air and fuel. CLC systems combust carbonaceous or hydrogen fuels by using a solid compound (typically a metal oxide) as an oxygen carrier. The metal oxide is circulated between two reactors: for combustion and regeneration.
In the combustion reactor, the fuel is oxidized by the oxygen carrier, which undergoes a corresponding reduction in the endothermic reaction. Because carrier-borne oxygen rather than air is used, it is sometimes called “air-independent oxidation.”
The oxygen-depleted carrier is then regenerated in another reactor, typically by exposure to air. The exothermic regeneration process restores the carrier to an oxygen-rich state, enabling its reuse in combustion.
By-products from the combustion reactor are water and carbon dioxide. When the steam is condensed, a fairly pure stream of CO2 is available for liquefaction, transport, and sequestration. The overall system function is similar to a conventional combustor, with the advantage that the output flow is free of nitrogen and excess oxygen. Because it does not require additional separation units, CLC technology avoids the energy penalty that traditional fossil fuel-fired combustors must pay to produce pure carbon dioxide. In addition, hot air from the regeneration reactor yields power through a thermodynamic cycle.
In alternatives to CLC using boilers, air is introduced by fans or other means to a combustion chamber. Fuel is also introduced to this chamber via pumps or other means. The chamber may be at or near atmospheric pressure or it may be pressurized. In the most common type of atmospheric boiler, just prior to entering the boiler combustion chamber, the air and the fuel are mixed in a burner.
The hot gases from combustion are nitrogen, carbon dioxide (the primary greenhouse gas or “GHG”), and water vapor along with pollutants such as nitrogen oxides formed from the extraneous nitrogen introduced with the oxygen needed for combustion (by volume air is 80% nitrogen and only 20% oxygen), sulfur oxides formed from fuel contaminants, and carbon monoxide due to incomplete combustion. The water vapor comes both from atmospheric humidity and from combustion. The water from combustion carries with it a portion of the fuel energy which can only be regained by condensing it to liquid.
The hot post-combustion gases are carried up by their buoyancy and pass through various heat exchange systems that boil the feedwater forming steam. Other heat exchange means superheat the steam. The cooled exhaust gases are then treated or exhausted to the atmosphere.
In an atmospheric fluidized bed boiler, the process is the same except that the fuel and air are mixed in, and combustion occurs in a bed of solids which is fluidized by their passage. A pressurized fluidized bed boiler is similar except that the entire process is contained in a pressure vessel, and the entering and exiting stream are pressurized. The pressurization reduces the volume of the gases and therefore the size of the equipment needed.
Existing external combustion boiler technologies have numerous problems and shortcomings, many related to extraneous nitrogen involved. The nitrogen: 1) requires major components (ducts, fans, the boiler itself, post combustion pollution treatment equipment) to be greatly oversized; 2) requires energy to supply it to the process (especially for a pressurized process); 3) carries energy away in the exhaust, as explained more fully below; and 4) results in pollutant (nitrogen oxides) formation.
“Oxy fuel” combustion is a prior art technology that partially addresses the issues of energy waste via exhaust. In this process, oxygen from an air separation plant is supplied to the combustion. Two types of air separation processes are cryogenic and pressure swing adsorption (PSA).